[slctech]

There is no need to be using 350psi for nitrogen testing purposes. Step it back to 150 and if in doubt, when leak checking, to 200psi maximum. Figure new condensers with the service valves isolating the compressor you will not do much harm at 350psi, but that much pressure across a compressor can do some internal damage, that is what compressor manufactures state, they do not want anymore than 250psi on the discharge line. Anyways it is just a waste of nitro.

As for drop in pressure, it could possibly be the ambient air temperature. There are a few long-winded threads on this forum about nitrogen pressure rising or falling based on the temperature of the air. Hotter it gets the nitrogen pressure will rise and as the afternoon into the evening cools, the nitrogen pressure will drop. Not going to take much at 350 psi to raise or drop the nitro load a few psi.

Sounds like your on the right track, kudos for purging and brazing with nitro, just step the test pressure back.

 

[slctech]

I promise you that 150psi for 90% of your residential test pressure procedures is more than plenty. I guess if your pressure testing a lone refrigerant line-set with a compressor isolated from any chance of seeing that high of pressure you will be fine.